This invention relates to a process and device for generation of a random signal. The invention is particularly applicable to the digital-analog conversion field and to the analog-digital conversion field. Consequently, the invention also relates to a digital-analog conversion system using such a random signal. It is applicable particularly for direct digital frequency synthesis, for example in the field of radar techniques or instrumentation.
Conversion devices (either digital-analog or analog-digital) are very widely used in many systems and their performances are usually a critical point in these systems, for example as illustrated by direct digital synthesis.
Direct digital synthesis is a frequency synthesis technique that consists of generating digital values of samples of a signal that is to be generated and converting these samples into analog signals using a digital-analog converter. Signal synthesizers made using this technique are very attractive in terms of volume, weight and energy consumption, since they can benefit from large scale integration. Their other advantages are particularly excellent resolution and very low switching times from one frequency to another. However, when a useful signal is passed through the digital-analog converter, it is accompanied by the creation of parasite signals due to non-linearities of these converters. These non-linearities are due to the fact that not all steps in the digital-analog converter transfer function are the same height and that the transition between steps produces irregular phenomena.
The same problem occurs in applications based on analog-digital converters in which the passage of signals in these converters is accompanied by the creation of parasite signals due to non-linearities.
It is known that adding a random signal to the useful signal before passing it through the converter is a means of reducing the level of parasite signals by reducing the effects of converter non-linearities mentioned above. This random signal is frequently called  less than  less than dither greater than  greater than . The useful signal usually has a limited band width and the clock frequency of the system, for example a digital synthesizer, is usually greater than this band. This leaves an empty spectral space in which the random signal can be placed.
In order to be fully efficient, this random signal must have some specific characteristics. Firstly, its spectrum must be controlled so that it does not encroach the useful signals band. Secondly, the quality of the converter linearization depends on the histogram of time amplitudes of the random signal. For example, the linearization achieved using a Gaussian law is not as good as can be obtained with a rectangular law. Therefore, there is a real advantage in being able to control the spectrum and the histogram at the same time, for the random signal.
Methods are known for obtaining a random signal with a given spectral envelope. Methods are also known for obtaining a random signal with a given amplitude distribution law. In particular, these methods are described in books describing probability calculations, for example such as the book entitled:  less than  less than Simulation dxc3xa9terministe du hasard greater than  greater than  (( less than  less than Deterministic simulation of chance greater than  greater than  by J. Maurin, published by Masson. However, there is no known method of creating a random signal when the spectral envelope and the amplitude distribution law are imposed simultaneously.
In particular, the purpose of the invention is to enable the construction of a random signal when the previous two parameters are imposed on it, in other words:
the spectral envelope of the signal, which is actually the modulus of the Fourier transform of its correlation function;
the time amplitude distributions law that will subsequently be called the amplitudes histogram.
Consequently, the purpose of the invention is a process for the generation of a random signal, characterized in that it comprises at least:
a first noise generation step;
a second noise filtering step to obtain a signal x(t) with a predetermined spectral envelope H(f);
a third step, in which a non-linear function g is applied to the signal x(t) in order to give a signal y(t) similar to a predetermined amplitudes histogram f(y), the function g being defined by the following relation:   y  =            g      ⁡              (        x        )              =          α      ⁢                        ∫          0          x                ⁢                                            P              ⁡                              (                u                )                                                    f              ⁡                              (                u                )                                              ⁢                      ⅆ            u                              
where the function P is a histogram of the signal x(t) to which the third step is applied and xcex1 is an amplitude adjustment factor that depends on the required amplitude for the signal y(t);
a fourth step in which a pulse response filter w(t) is applied to the signal y(t) to correct its spectral envelope and obtain an output signal s(t) with a predetermined spectral envelope H(f), the pulse response w(t) being the inverse Fourier transform of a frequency function W obtained by dividing the function H(f) by the modulus Y2(f) of the Fourier transform of the signal y(t), and then multiplying by a constant xcex2.
Another purpose of the invention is a device for making use of the above-mentioned process and a digital-analog conversion system using a random signal generated according to this process.
The main advantages of the invention are that it improves the linearity of analog-digital and digital-analog converters, that it can be applied to many systems, and is economic and easy to use.